This invention relates to a catalytic combustor for a combustion turbine and, more specifically, to a piloted rich-catalytic lean-burn hybrid combustor having a plurality of cooling air conduits passing through a fuel/air mixture plenum.
Combustion turbines, generally, have three main assemblies: a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor compresses ambient air. The compressed air flows into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited creating a heated working gas. The heated working gas is expanded through the turbine assembly. The turbine assembly includes a plurality of stationary vanes and rotating blades. The rotating blades are coupled to a central shaft. The expansion of the working gas through the turbine section forces the blades, and thereafter the shaft, to rotate. The shaft maybe connected to a generator.
Typically, the combustor assembly creates a working gas at a temperature between 2,500 to 2,900 degrees Fahrenheit (1371 to 1593 degrees centigrade). At high temperatures, particularly above about 1,500 degrees centigrade, the oxygen and nitrogen within the working gas combine to form the pollutants NO and NO2, collectively known as NOx, a known pollutant. The formation rate of NOx increases exponentially with flame temperature. Thus, for a given engine working gas temperature, the minimum NOx will be created by the combustor assembly when the flame is at a uniform temperature, that is, there are no hot spots in the combustor assembly. This is accomplished by premixing all of the fuel with all of the of air available for combustion (referred to as low NOx lean-premix combustion) so that the flame temperature within the combustor assembly is uniform and the NOx production is reduced.
Lean pre-mixed flames are generally less stabile than non-well-mixed flames, as the high temperature regions of non-well-mixed flames add to a flame""s stability. One method of stabilizing lean premixed flames is to react some of the fuel/air mixture in a catalyst prior to the combustion zone. To utilize the catalyst, a fuel/air mixture is passed over a catalyst material, or catalyst bed, causing a pre-reaction of a portion of the mixture and creates radical which aid in stabilizing combustion at a downstream location within the combustor assembly.
Prior art catalytic combustors completely mix the fuel and the air prior to the catalyst. This provides a fuel lean mixture to the catalyst. However, with a fuel lean mixture, typical catalyst materials are not active at compressor discharge temperatures. As such, a preburner is required to heat the air prior to the catalyst adding cost and complexity to the design as well as generating NOx emissions, See e.g., U.S. Pat. No. 5,826,429. It is, therefore, desirable to have a combustor assembly that burns a fuel lean mixture, so that NOx is reduced, but passes a fuel rich mixture through the catalyst bed so that a preburner is not required.
One disadvantage of using a catalyst is that the catalyst is subject to degradation when exposed to high temperatures. High temperatures may be created by the reaction between the catalyst and the fuel, pre-ignition within the catalyst bed, and/or flashback ignition from the downstream combustion zone extending into the catalyst bed. To reduce the temperature within the catalyst bed, prior art catalyst beds included cooling conduits which pass through the catalyst bed. The cooling conduits were free of the catalyst material and allowed a portion of the fuel/air mixture to pass, unreacted, through the cooling conduits. Another portion of the fuel/air mixture passed over, and reacted with the catalyst bed. Then, the two portions of the fuel/air mixture were combined. The unreacted fuel/air mixture absorbed heat created by the reaction of the fuel with the catalyst and/or any ignition or flashback within the catalyst bed. See e.g., U.S. Pat. No. 4,870,824 and U.S. Pat. No. 4,512,250.
The disadvantage of such cooling systems is that the cooling conduits utilize a gas comprising a fuel/air mixture. This fuel/air mixture is subject to premature ignition within the cooling conduits. Such premature ignition would destroy the heat absorbing capability of the fuel/air mixture thereby allowing the catalyst bed to overheat.
There is, therefore, a need for a catalytic reactor assembly for a combustion turbine, which includes a cooling means that does not rely on a fuel/air mixture to be a cooling fluid.
There is a further need for a catalytic reactor assembly for a combustion turbine, which eliminates the possibility of igniting the gas within a cooling passage.
There is a further need for a catalytic reactor assembly which improves the performance of the catalyst to a point where a preburner is no longer required.
There is a further need for a catalytic reactor assembly which maybe retrofitted with existing combustor designs.
These needs, and others, are satisfied by the disclosed invention which provides a catalytic reactor assembly having a fuel/air plenum with cooling conduits passing therethough. The cooling conduits are in fluid communication with an air source. The outer surface of the cooling conduits and the inner surface of the fuel/air plenum are coated with a catalytic material. The fuel/air plenum and the cooling air conduits each have a downstream end which is in fluid communication with a mixing chamber. Thus, a fuel rich fuel/air mixture may pass through the fuel/air plenum. Air passes through the cooling conduits. When the fuel/air mixture and the cooling air are mixed, a fuel lean pre-ignition gas is created. The fuel lean pre-ignition gas is ignited creating a working gas with a reduced amount of NOx.
The fuel/air plenum is created by an inner shroud and an end plate which is located opposite the downstream end of the fuel/air mixture plenum. A first plenum surrounds the fuel/air plenum. The first plenum is in fluid communication with a fuel source and an air source. The air source may be the same source which provides air to the cooling conduits. At the downstream end of the mixing chamber is a flame chamber and igniter assembly.
The catalytic reactor assembly may be included in the combustor assembly of a combustion turbine which includes a compressor assembly, a combustor assembly and a turbine assembly. Typically, the combustion turbine includes an outer shell which encloses a plurality of combustor assemblies. The outer shell creates a compressed air plenum which is fluid communication with the compressor assembly. At the downstream end of the combustor assemblies are transition sections, which are also enclosed within the compressed air plenum, which are coupled to the turbine assembly.
It is advantageous to have a fuel rich mixture in the catalyst section for several reasons. For example, the catalyst is more active because more fuel is in contact with the catalytic material. This allows the catalyst to be active at temperatures below the temperature of the air at the exit of the compressor. Therefore a pre-burner is not required upstream of the catalyst to preheat the fuel/air mixture. Additionally, having an oxygen lean environment in the catalyst zone controls the amount of fuel that is reacted. When less fuel is reacted, less heat is created therefore limiting the temperature in the catalyst bed.
In operation the compressor assembly compresses ambient air which is delivered to the compressed air plenum. Compressed air within the compressed air plenum is split into at least two portions: the first portion enters the first plenum and the second portion travels through the cooling conduits. A third portion may be directed to an pilot assembly. Within the first plenum, a fuel is introduced from a fuel source and mixed with the first compressed air flow to create a fuel rich fuel/air mixture. The fuel rich fuel/air mixture is delivered to the fuel/air plenum which surrounds the cooling air conduits and is in contact with the catalyst material. The fuel rich fuel/air mixture is reacted with the catalyst material and delivered to the mixing chamber. The second portion of compressed air enters the cooling chambers and absorbs heat from the catalytic reaction. The second portion of the compressed air then passes into the mixing chamber where it is mixed with the heated fuel/air mixture to create a pre-ignition gas. The combined pre-ignition gas contains an excess of air and is, therefore, fuel lean. The fuel lean pre-ignition gas is delivered to a flame zone where it auto-ignites or is ignited by the pilot assembly creating a working gas. The working gas travels through the transition sections and is delivered to the turbine assembly.